Black iron pipe fittings are cast and rated for up to 600 psi.
Similar looking forged hydraulic fittings are rated for 3000 psi minimum.
The noticeable difference between the 2 types is the parting line. Cast fittings have a narrow parting line and forged fittings have a wide parting line. There is also a substantial difference in appearance between the relatively smooth surface of the forged fittings and the rough texture of sand casted black iron, and the bulk of forged fitting when compared to the same size of cast fitting.
WOG (water, oil, and gas) black iron fittings are only acceptable for lines between the reservoir and the pump and for the return lines from the system to the reservoir. All pressure lines should be pressure rated hydraulic fittings, even if the system pressure is on the low side. Impact on hydraulic components such as front blade angling cylinders can drive the pressure far beyond 600 psi.
Depending on the location of a broken fitting it may not be noticeable from the operators station until the limited supply of fluid has been dumped on the ground leaving the tractor stranded. Most GT's with on board hydraulics have from 5-11 quarts in the rear end, and the charge pumps will move from 12 - 17 quarts per minute. The loss of as little as 3 quarts is enough to cause the hydro to be non-functional. Auxiliary loader hydraulic systems have flow rates up to 24 quarts per minute and typically have 8 quarts in the reservoir.
A catastrophic failure of a black iron fitting on a pressure line can have your machine down hard in 15-30 seconds if it goes unnoticed. Even a cracked fitting that only loses a drop every second or two will cause functional errors in a couple of hours of constant work.
That last is from practical experience. The operator rarely notices drops of hydraulic fluid on the ground when doing dirt work.